As machine-to-machine (M2M) communications technologies rapidly develop, a market requirement and scale explosively grow in recent years.
A terminal with an M2M service encounters a low-consumption and low-cost challenge. Crystal oscillator accuracy of the M2M terminal is relatively low due to a cost limitation. Consequently, the M2M terminal has a relatively large frequency offset relative to a communication peer end (such as a base station). A signal phase rotation may be generated in a time domain due to the frequency offset.
In a Long Term Evolution (LTE) system, a Zadoff-Chu (ZC) sequence is used as a primary synchronization sequence, to perform symbol timing synchronization and carrier frequency offset estimation.
UE implements symbol timing synchronization by using an autocorrelation feature of the ZC sequence, that is, determines a symbol position and an optimal sampling point by using a position in which a correlation peak is generated when a sliding correlation is performed. The method has a precondition, that is, in sequence duration, signal phase deflection (generated due to frequency offset existence and a relative movement between sending and receiving ends) cannot be excessively large, for example, cannot exceed π.
Currently, the crystal oscillator accuracy of the M2M terminal is generally 20 parts per million (PPM). In a case in which a carrier frequency in the LTE system is 2 GHz, 20-PPM crystal oscillator accuracy means that the frequency offset is approximately 40 kHz. If an example in which a symbol rate in the LTE system is 15 kHz is used, a phase in one symbol may rotate by 40/15×2π, which is close to 6π. However, a signal generally occupies at least one symbol in a time domain. Consequently, grid search needs to be performed multiple times to compensate for such a large phase rotation, because when the frequency offset is excessively large, the terminal can only eliminate frequency offset impact by performing blind tests on different frequency offset values. An example in which a frequency interval of the grid search is 50 Hz is used, and the grid search may be performed 800 times due to the foregoing 40 kHz frequency offset. Consequently, implementation complexity of the M2M terminal greatly increases.
In conclusion, for a terminal with relatively low crystal oscillator accuracy, such as a current low-cost M2M terminal, a relatively large phase rotation is caused due to a frequency offset, grid search needs to be performed multiple times to compensate for the relatively large phase rotation, and implementation complexity of the terminal is relatively high.